When being energized, a motor can generate a predetermined torque at a rotational shaft to rotate the shaft, and also controlling the position of the rotation. However, when no power is supplied, the motor cannot control the rotation of the rotational shaft, and thus if the rotational shaft is subject to an external torque, the rotational shaft easily rotates. Thus, there is a need for a device whose stop position is to be fixed accurately even when no power is supplied such as a tape feeder unit to include a rotational position holding device such as an electric-powered brake device and an inverse input clutch.
For example, there is an electric-powered brake device as described below. At the electric-powered brake device, when being not energized, a brake pad is pressed against a rotational member by a biasing force of a spring to restrain the rotational member. When being energized, the restrained state is canceled out by an electromagnetic force of an electric magnet against the biasing force.
In addition, the inverse input clutch transmits a rotational force of an input-side member to an output-side member, by the use of a lock mechanism that engages an engagement piece into a wedge-shaped portion between a stationary member and the output-side member and disengages the engagement piece from the wedge-shaped portion, as in the invention described in Patent Document 1, for example. The inverse input clutch locks the output-side member when the output-side member is subject to an external rotational force.
However, the former conventional art (electric-powered brake device) constantly consumes electric power while the brake is released besides during motor operations, thereby resulting in decrease of energy efficiency. Further, the addition of the brake device makes the entire device larger in size.
The latter conventional technique (inverse input clutch) is complicated in structure because a rotational force is transmitted by engagement of a pin (3b1) with a pin hole (2b3) into which the pin is inserted and a cylindrical portion (3c) for pressing and moving an engagement piece (20) is provided separately from the pin, and the like (refer to Patent Document 1, FIGS. 16 to 18). It is thus difficult to reduce the device size of the inverse input clutch to an extent that the internal diameter of a housing chamber becomes about 12 mm, for example. In addition, the conventional inverse input clutch needs to have a relatively smaller angle of the wedge-shaped portion of 3 to 4.5° (refer to Patent Document 1, paragraph 0012). Thus, if the spacing between two engagement pieces (20) is widened to reduce the diameter of the engagement pieces, for example, the angle becomes larger and the engagement pieces do not come to engage into the wedge-shaped portion. This also constitutes a hindrance to downsizing the engagement pieces and the entire structure.